TRBUSNICA, Serbia — Police from Serbia and the Serb mini-state in Bosnia have held a joint anti-terrorism drill that could add to simmering tensions in the Balkans stemming from the 1990s war.

The exercise, dubbed “Drina 2016,” was held Sunday at the border between Serbia and Bosnia simulating an extremist attack and a bus hijacking. Several thousand people watched the drill.

Serbian President Tomislav Nikolic says that the exercise has shown that borders shouldn’t exist when it comes to fighting terrorism.

Joint security drills by Serbian and Bosnian Serb troops remain a sensitive issue because Serbia backed Bosnian Serbs in their fight against Bosnia’s Muslims and Croats during the 1992-95 war.

About 100,000 people died in the war, and millions were made homeless amid numerous atrocities.